I struggle a lot with this issue, and couldn't find anything to help me so I hope here could:

I did set up an event, with different fees, and one of them is free. When people check not free, everything is going fine, but when they check free, they are not redirected correctly after "confirm your choice": the page is bugged and they do not receive any confirmation email. They are redirected on an error page (HTTP ERROR 500)
So, even though they are registered, they cannot see it and try again and again, registering each time.
The payment method is Paypal.

The CMS I am on is Joomla, and I use version 4.7.25 of CiviCRM.

This is a big problem and I don't see any way out, so thank you for your help.